The use of mechanical or manual, oscillating or rotating devices to agitate and mix liquids has been known for centuries. The majority of these prior art methods involve mechanical devices or systems that intrude into the liquid volume or mass. Agitation of a liquid is usually achieved by a mechanical propeller or similar rotating or oscillating device that induces a flow which in turn causes mixing. Another approach for mixing is to continuously pump the liquid into an external system which introduces other materials into the liquid. The action of the pump and the natural turbulence of the flow through pipes and orifices agitates the liquid.
These oscillating and propelling devices require mechanical hardware which intrude into the liquid mass. In externally pumped systems, the propelling hardware operates outside the main volume but requires external plumbing to convey the liquid to and from the agitation. Furthermore, the mechanical device is usually driven by a motor which require the use of bearings and seals. The seals wear and often leak, and the bearings need lubrication creating a need for monitoring and servicing over long periods. For space applications, the agitation systems are frequently inaccessible and unserviceable, particularly on unmanned flights.
In industrial systems, the use of mechanical devices with their attendant bearings and seals are subject to attack, particularly if the liquid contains abrasives or is chemically aggressive. Furthermore, the mechanical approach often introduces contaminants into the liquid from leaking seals, bearing lubricants, and particle contamination due to wear. The same mechanical parts usually complicate the cleaning of fluid systems between uses.
The presence of unwanted bubbles and solids in liquid masses often creates a problem of removal, typically accomplished by using mechanical filters or centrifuging to remove these unwanted objects from the liquid. Other methods including gravity or buoyancy are often used to separate the solids and bubbles from the liquid. A settling tank is an example where the flow of the liquid is temporally halted so that relatively dense objects will migrate toward the bottom, while relatively light objects such as bubbles will gravitate to the top of the liquid.
Yet another approach employs the use of electric or magnetic fields to act on charged particles to cause their separation from the liquid.
These various prior art separation systems all have certain drawbacks. For example the mechanical filters used to trap particles and bubbles, requires movement of the liquid through the filter. To filter the entire volume of liquid, the entire volume must pass through the filter. Filtering of a small mass of liquid requires that the liquid be transported from one container to another by pouring or by pumping the liquid through the filter.
In space, gravity cannot be relied upon to separate particles or bubbles from a fluid. In earth applications, gravity has little influence on objects that have a density close to that of the liquid. Buoyancy likewise is not effective for separation when turbulent flow acts to keep objects and bubbles suspended in liquid. Centrifugal segregation can be used in space but its effects cease when rotation stops. Furthermore, the centrifugal rotation of complex and sometimes large liquid volumes, and the associated machinery needed for centrifugal rotation creates added complexity for the space driven application.
The deployment of drops and gas or gas bubbles in liquid typically require the use of a needle to control placement of the drop or gas bubble. The drop or bubble is deployed at very low velocity to prevent splatter or dispensing of excess material. A volumetric pump or a pressure driven displacement mechanism is used to form the drop. Gravity or inertia is used to separate the drop; alternatively, the drop is dispensed by direct wetting against a target surface.
In space applications, matched needles are often used to form a drop between them. Retraction of the needles at high speed causes the drop to be left in a stationary position within the gas or the liquid media. However, the use of needles for deployment of drops and bubbles in space is very restricted. In space, the inertia of the drop is used to overcome surface tension when the matched needles are retracted. The need to overcome surface tension is a property of the material that is not easily controlled, and thus limits the useful application of this form of dispensing.
On earth, the gravity is a force that controls the drop size and deployment. The drop deployment occurs when the drop grows until its weight exceeds the surface tension. Again, the deployment relying upon the weight of the drop, is not easily controlled, thereby limiting the use of this method for dispensing drops and bubbles.
Free surface manipulation such as the formation of standing waves for coatings or for mass solder operations such as "wave soldering", usually rely on a liquid jet. A stream of liquid is propelled from the bottom of a free surface pool. The stream is directed at the surface and creates a small fountain which appears as a standing wave.
The mechanical method of manipulating free liquid surfaces share problems with mechanical agitation. Here again, the presence of seals, bearings and their intrusive nature creates serviceability and maintenance problems and limits their effectiveness. The inability to sense the position of the surface and to respond to changes is a further limitation that prevents the widespread use of mechanical means for surface manipulation.
Manipulation or the act of controlling the position of an object in a liquid is usually done with intrusive mechanical means. Probes and needles are used to propel objects and control their placement. Liquid jets from nozzles can also be used for this purpose.
As with the prior art drawbacks with agitation, the intrusive mechanical manipulation of immersed objects likewise has the ongoing problem with seals, bearings, mechanical linkage, leakage and wear servicing. Furthermore, the limited mobility of mechanical devices restricts the ability to manipulate objects that may move outside the working range of the mechanical device. Mechanical devices lack the ability to sense and to provide feedback to control the manipulation without a great deal of hands on intervention.
Common spraying techniques produce large numbers of droplets. Where the droplets are required to be uniform in size, ink jet printing is a common method of achieving this objective. In some embodiments, a steady stream of droplets are emitted from the orifice, and capillary waves are induced by external vibration or pressure perturbation. In another embodiments, a small volume of liquid is contained in a reservoir equipped with an orifice or nozzle that is juxtaposed to a pressure pulse source. The pressure source emits a mechanically or thermally induced pressure wave that causes a high pressure transient at the orifice which thereby ejects a small volume of liquid as a drop.
These spraying processes produce a wide distribution of droplets at high velocity and over a wide pattern. In order to be used to apply coatings, paints and adhesives to specific areas on a surface, it is necessary to mask or shield all the areas that are not intended to receive the liquid. This causes the liquid that falls on the masked area to be lost, thereby incurring waste in terms of liquid and masking material.
Jetting techniques such as those used for ink jet printers, operate without masks but have a very small orifice or nozzle diameter, usually smaller than the drops which are produced. The smaller orifice is easily clogged by solid particles. The continuous ejection mode has a narrow operating range which normally requires an ejection velocity of several meters per second. To produce uniform drops, the process of ejection must keep up with the jet velocity which in turn means that the droplet production rate may be as high as 100,000 drops per second or greater. This high production rate makes it difficult for most existing systems to use all of the drops. The efficiency of some ink jets may be as little as two percent of the drops produced, thereby necessitating collection and recycling of the remaining ninety-eight percent of the drops.
The drop-on-demand drop ejectors produce only the drops needed at a given instant, thereby requiring the pressure of ejection to occur instantaneously. The high pressure transient may be absorbed easily by a gas bubble in the system, therefore bubbles cannot be tolerated in the system. The drop-on-demand system is not practical with high viscosity liquids.
Simple focused acoustic beams in burst mode can achieve droplet ejection in a wider range of operating conditions than the prior art methods and can be adjusted during operation. One limitation with respect to this procedure is the sensitivity to liquid surface position relative to the focal length. The liquid surface must be within a wavelength of the maximum intensity to achieve drop ejection. As a result, the process is sensitive to surface waves and variations in the surface position.